The invention relates to tread rubber compounds made employing a high structure carbon black as a reinforcing material, and a pneumatic tire made using tread rubber which is reinforced using the carbon black.
Irregular treadwear on tires is a relatively recent problem which has been caused, in part, by the requirements imposed by recent changes in vehicle construction. Vehicles with broad specifications for both camber and toe-in, and front wheel drive vehicles, are particularly prone to causing irregular wear on tires. Front wheel drive vehicles, in comparison to the traditional rear wheel drive vehicles, tend to quickly wear out the drive tires and have a very slow treadwear rate on the free rolling tires. This slow wear rate on the free rolling tires, in combination with other factors, proves to quicken the onset, and magnify the relative amount of irregular treadwear.
Chassis designs with independent swing axle suspensions are also found to increase the possibility or severity of irregular wear.
Further complicating the problem is the fact that in the competitive tire industry, it is a goal to provide tires having treads that demonstrate good all weather traction and have distinctive designs. Because of design considerations, it is sometimes desirable to provide tread lugs of a length that traverses several zones of the footprint area of the tire. Because different stresses or forces are encountered by different zones of the footprint when a tire is in motion, depending on whether the zone is in the shoulder area, equatorial plane, near the leading edge of the footprint, the trailing edge of the footprint or the middle of the footprint, it has been discovered that some tire designs may be especially subject to irregular wear.
It has been theorized that in some tire designs irregular wear occurs when a leading edge of a relatively long tire lug is free of the footprint area of the tire while a second portion of the tread lug is still in the footprint area of the tire, and the different stresses encountered cause a distortion of the tread lug; and when the portion of the lug under compression starts to come out of the footprint area, the original shape of the lug is restored with a snap, and the motion or squirm associated with the snap wears the trailing portion of the lug against the road surface. It is also possible for wear to occur at the leading edge of the footprint because of the initial compression of the lug in the footprint.
In order to avoid this problem, it was theorized that a tread rubber compound that has a high stiffness (to minimize squirm of the lug in the footprint area) yet had good rolling resistance properties, while maintaining good traction could be developed by choosing the right combination of rubber and rubber fillers and reinforcers.
It is proposed, in accordance with the present invention, to optimize rubber reinforcement to address this problem.
It is well known that high surface area, small particle size carbon blacks provide greatest reinforcement in an elastomer, as evidenced by improved resistance to abrasion, tensile strength and tear properties: concomitantly, superfine blacks also provide higher hysteresis and poorer dynamic performance attributable to high heat build-up, both adverse properties. In addition, it is known that larger size blacks provide reduced rolling resistance and poorer traction, both wet and dry.
In the recent past, the emphasis has been on reduction of rolling loss for lower fuel consumption without sacrificing wear resistance. Ignoring changes in tire construction to aid in this respect, several approaches have been taken towards attaining this goal, as taught in U.S. Pat. Nos. 3,824,206; 4,224,197 and 4,281,703, inter alia: and United Kingdom patent applications Nos GB 2,082,486A and 2,057,455A.
U.S. Pat. No. 4,644,988 is concerned with deliberately increasing the rolling resistance and heat buildup in a tire.
The presence of a black in tread rubber also provides a hysteretic component essential to traction, as do other reinforcing ingredients such as silica. Although there are certain correlations between black properties and the properties of tread rubbers using the black that can be made, there is no certain way known to predict the ultimate effects of the use of a particular black in tread rubber, which effects must therefore be determined or confirmed by experiment. The choice of the type and amount of carbon black used in the tread compound influences many performance properties of a tire. N100 to N300 blacks are used in tires where excellent abrasion resistance and tear properties are important, as for example in truck and passenger tires for all types of vehicles.
The distinction between grades of carbon blacks are based on three main factors which are broadly classified as follows:
1. Particle size, particularly as it relates to surface area.
2. Structure, a general measure of particle-to-particle association, or, i.e. the strength of the aggregate.
3. Chemical composition of the surface, or surface activity.
Surface area (1), generally measured by iodine number ("I.sub.2 No."), and structure (2), generally a measure of void volume, in turn measured by dibutyl phthalate absorption ("DBPA"), are key attributes which characterize a carbon black.
This invention relates to the use of particular tread carbon blacks, having very high structure, in tread rubber compounds.
A detailed discussion of some of the foregoing considerations is presented in an article titled "The Effects of Carbon Black and Other Compounding Variables on Tire Rolling Resistance and Traction" by W. M. Hess and W. K. Klamp, Rubber Chemistry and Technology, Vol. 56, No. 2 May-Jun. 1983. A predominately SBR tire tread with minor amounts of polybutadiene rubber (BR) essentially all of which is cis-, and/or natural rubber (NR), and an oil loading of typically about 70/40 black/oil ratio with a high structure N220 black, was used to study the effects of carbon black and other variables on tire tread wear, rolling resistance and traction, among other objectives. The blacks tested varied in nitrogen surface area (N.sub.2 SA), ASTM tint, and DBPA.
In the prior art, carbon black used in tread compounds has a DBPA in the range of 110-130 cm.sup.3 /100 g and an I.sub.2 number in the range of 65-110 mg/gm. Tires with good rolling resistance properties, in general, use tread carbon black having properties in the low end range of the I.sub.2 numbers (large size). Slight rolling resistance advantages are seen in tires made with carbon blacks having a DBPA of 130 cm.sup.3 /100 gm (higher structure) when compared with tires made with blacks having a DBPA of 110 cm.sup.3 /100 g.
It has also been recognized that the physico-chemical nature of the carbon black particles' surface (3) (surface activity), and particularly, the nature of the carbon atoms at the surface of a particle, may affect rubber reinforcement. Similarly the chemical nature of the particles' surface, and particularly the presence of oxygen at the surface along with phenolic, ketonic and carboxylic groups, inter alia which are known to be present on the surface, affect the crosslinking of the rubber and its vulcanized properties. See Rubber Technology and Manufacture, edited by C. M. Blow, p 180, CRC Press, International Scientific Series (1971). According to Blow, there appears to be no direct connection between the chemical nature of the surface (i.e. the individual chemical groups of the particle surface) and the surface activity or the properties which the black confers on the rubber.
West et al showed that an increasing rolling loss coefficient, caused by increasing the nitrogen surface area of the black (N.sub.2 absorption), gave a correlation coefficient of 0.97 for N234, N351, N375 and N220 blacks. A similar correlation was made regarding the I.sub.2 No. and tinting strength of the blacks. No such meaningful correlation was found regarding the compressed DBPA values of the blacks. (It should be noted that the blacks used in the study have similar DBPA values (in the range of 114-125), ad it would be expected that any differences in properties based on DBPA or crushed DBPA would be difficult to detect for these blacks.) Although this apparently contradicts what was known in the prior art, West's dry traction results showed a poor correlation with rolling loss. West's dry traction results did correlate well with I.sub.2 No. and tinting strength (see "The Effects of Carbon Black on Rolling Resistance of a Tire Tread Compound" by J. R. West et al, Rubber Chemistry and Technology, supra, at page 509).
The ratio of N.sub.2 absorption to I.sub.2 absorption (N.sub.2 /I.sub.2) can be used as a measure of surface activity. A ratio greater than 1 denotes high surface activity.
Other methods of measuring surface activity, e.g. the bound rubber test, are well known in the art.
It is an object of the invention to use property correlations of carbon blacks in general to provide a rubber composition which is reinforced with high structure carbon black and used in the tread rubber of a tire, which provides a tire which demonstrates good irregular wear properties and good rolling resistance properties while maintaining other necessary properties.
Other objects and uses of the rubber compositions of the invention will be apparent to those skilled in the art from the following description and claims.